Basically its simple situation, when popular star left show, which gave him popularity. Different reasons as money or interest, but here is list of 15 people, who ended their career in popular TV-shows.
Last February, America Ferera reported, that she leaves “Superstore.”

That happened after 5 seasons. She said that past years were really amazing and memorized time of her professional career, great teamwork gave her chance to become popular actress, but now it’s enough for her, she will not take part in season 6.
That lasted 15 years, Justin Chambers was playing Alex Karev in ABC “Gray’s Anatomy”.

He was playing it since 2005, and was only one person that continued playing after first season. This is sad situation for him, as he says, he can’t find right way to left it, because it was everything in his life for 15 years. Now he is 51, and want to stay more with his kids and wife. He just filmed last scenes, to remove his character from the film.
Sket Ulrich does not want to continue filming in season 5 of “Riverdale”

Skeet Ulrich, was playing F.P. Jones on “Riverdale” for 4 years, said he stops doing that in 2020. As he says, he lost all interest in this project, but there is a small chance of comeback to the show.
One year ago, Nicole “Snooki” Polizi and her manager reported, she stops her career in the “Jersey Shore: Family Vacation.” Later after report, she asked do not blame her decision, because it’s best option for this moment. She will not play in next season, she just can’t do it anymore, tired enough to stop playing. He tried to left show 3 times before and finally did it, now she can spend more time with her children and husband. Show started 11 years ago.
Kal Penn stopped playing in “House”, because he wants work in politics.

He played Dr. Lawrence Kutner on “House” for two years, after that he stopped working in TV-series and found new work in Obama administration as director of communications.
Jim Parsons left great TV-show “The Big Bang Theory” and because of this decision, show ended its life.

He worked for CBS more than 12 years, he earned about 51 million USD in last two seasons. He really misses his team and fans, saying thanks to all of them.
Mischa Barton is tired working as Marissa Cooper on “The O.C.”

They filmed 3 seasons, and that was enough for her to leave the show, as she’s saying, she opened her character as much as possible, now it’s time to end. Later she surprised everyone, that she did not really like that series.
Christopher Meloni stopped his career in “Law and Order: SVU” after 12 seasons.

Christopher Meloni finally said that this is right time to leave the show and continue his life without that TV series.
T.R. Knight decided to left “Grey’s Anatomy” after problem, that TV-show did not give enough time for his character. That happened after 5 years of filming in very popular show, because of problems in communication with management.
Dave Chappelle left TV-show called “The Chapelle Show” and a $50 million payment did not help, because “it was not that good”.

He decied to leave his own popular comedy show after 3 years. He said, it was good time, but he can’t do it anymore.
Adam Brody changed his position on “Gilmore Girls” to play in”The O.C.”

David Caruso stopped playing in “NYPD Blue” after 4 series.

He left show because of problems with payments. He asked 100 001 USD instead of 40 000 USD for one episode as they wanted to pay him.
Topher Grace did want to play in “That ‘70s Show” anymore, because she earned enough money in her life.

Jared Padalecki changed his career in “Gilmore Girls” for main character in “Supernatural.”
